The introduction of Third Generation (3G) communication systems will significantly increase the possibilities for accessing services on the Internet via mobile user equipment (UE) as well as other types of UE.
Various user equipment (UE) such as computers (fixed or portable), mobile telephones, personal data assistants or organisers and so on are known to the skilled person and can be used to access the Internet to obtain services. Mobile user equipment referred to as a mobile station (MS) can be defined as a means that is capable of communication via a wireless interface with another device such as a base station of a mobile telecommunication network or any other station. Such a mobile user equipment can be adapted for voice, text message, data communication or multimedia communication via the wireless interface.
The term “service” used above and hereinafter will be understood to broadly cover any service or goods which a user may desire, require or be provided with. The term also will be understood to cover the provision of complimentary services. In particular, but not exclusively, the term “service” will be understood to include Internet multimedia services, conferencing, telephony, gaming, rich call, presence, e-commerce and messaging e.g. instant messaging.
The 3G Partnership Project (3GPP) is defining a reference architecture for the Universal Mobile Telecommunication System (UMTS) core network which will provide the users of UE with access to these services. This UMTS core network is divided into three principal domains. These are the Circuit Switched domain, the Packet Switched domain and the Internet Protocol Multimedia (IM) domain.
The latter of these, the IM domain, makes sure that multimedia services are adequately managed. The IM domain supports the Session Initiation Protocol (SIP) as developed by the Internet Engineering Task Force (IETF).
SIP is an application layer signalling protocol for starting, changing and ending user sessions as well as for sending and receiving transactions. A session may, for example, be a two-way telephone call or multi-way conference session or connection between a user and an application server (AS). The establishment of these sessions enables a user to be provided with the above-mentioned services. One of the basic features of SIP is that the protocol enables personal mobility of a user using mobile UE by providing the capability to reach a called party (which can be an application server AS) via a single location independent address.
In this document the following abbreviations will be used:    AS Application Server    BGCF Breakout Gateway Control Function    CN Core Network    CPS Connection Processing Server    CS Circuit Switched    CSCF Call Session Control Function or Call State Control Function    DNS Domain Name System    ENUM See “E.164 number and DNS” (RFC2916)    FQDN Fully Qualified Domain Name    GW/S/AS network function or entity e.g. a proxy and/or Gateway and/or Server and/or Application Server or the like    HSS Home Subscriber Server    I-CSCF Interrogating CSCF    ID Identity    IM IP Multimedia    IMS IP Multimedia core network Subsystem    IMS-WV-GW Gateway between IMS and WV networks    IP Internet Protocol    ISC IP multimedia Service Control    MGCF Media Gateway Control Function    NAPTR Naming Authority Pointer (RFC 2915)    O-CSCF Outbound CSCF    P-CSCF Proxy CSCF    PMG Presence (P), Messaging (M) and Group Management (G)    PLS Presence List Server    PS Presence Server    PMG-WV-GW Gateway between IMS and WV networks    RR Resource Record of DNS    S-CSCF Serving CSCF    SIP Session Initiation Protocol (RFC 3261)    SIP URI SIP Uniform Resource Identifier (RFC 3261)    SLF Subscription Locator Function    SSR Service and Subscription Repository    TEL URL Is an URL associated to a terminal that can be contacted using the telephone network (RFC 2806)    UE User Equipment    UMS User Mobility Server    UMTS Universal Mobile Telecommunications System    URI Uniform Resource Identifier    URL Uniform Resource Locator    WV Wireless Village
Terminating sessions/transactions are routed in an IMS from the I-CSCF to an S-CSCF that can route them to an AS following the rules of a filter criteria. If the target identity (i.e. public user identity) is not registered, the I-CSCF selects an S-CSCF, and the S-CSCF down loads filter criteria from the HSS. However there is a problem where the target identity is not an IMS identity—non-IMS identities are routed over the IMS network to a non-IMS network.
An AS originated session/transaction is routed in IMS from AS to an S-CSCF that can route them further. Normally this S-CSCF is the one that was used when the session/transaction was routed from S-CSCF to AS, or address of the S-CSCF that is returned from the HSS or other database as response to a query, or address of the (default) S-CSCF may be configured in AS or fetched from an internal or external database, table, list, configuration data storage or alike. There are cases where it is difficult or impossible to find an S-CSCF.
Here are some examples where it can be difficult to find a S-CSCF:
a) If the subscriber is not registered, possibly no S-CSCF is assigned to the subscriber (or more accurately to any public user identity of the subscriber).
b) If the sending network element is a service server that routes a session/transaction on behalf of the user, there is a similar situation i.e. there may be no S-CSCF assigned to the user. (This kind of service server is referred to as user dependent service server).c) If a third party user uses a group identity as target address e.g. a message is sent to a group by a user that is not the “owner” of the group identity, there is a problem in deciding which S-CSCF should be used when the group server sends a message to each member of the group.d) If the sender is a service that has no connection to any user (i.e. the sender is a user independent service server). At least in this case the AS has to choose an S-CSCF or use a default S-CSCF. Both solutions have drawbacks. In the first one the AS has to perform functionalities of I-CSCF i.e. choose an S-CSCF. In the second one (i.e. if the default is used), the problem is how the load is balanced (Round robin functionality in DNS may be used.)
An additional argument against routing through an S-CSCF is that no service of S-CSCF is needed e.g. no filter criteria is utilized. This is especially true in the user independent service, server case.
Routing with service identities is another problem of IMS. In order to route to an AS, server, gateway, network function, network entity or alike that hosts or offers the service, an entry is needed in SLF and HSS containing routing information (e.g. filter criteria) for routing to S-CSCF and from S-CSCF to the correct AS, server, gateway, network function, network entity or the like that hosts or offers the service. The result is that HSS has to contain all service identities with proper-routing information. There is a similar problem with group identities created by users. A user may for example create a group of work colleagues, a group of family and a group of friends. These identities with proper routing information have to be included in HSS. Service identities may be quite stable but the group identities may be changed relatively often. A group identity may be a list of users that can be used e.g. to send a message to all of them with a single message sending procedure (instead of repeating the procedure in order to send the same message to every one of them). The problem of using a service and group identity is the creation/modification/deletion of a more or less temporary entry in HSS in order to make the routing possible via an S-CSCF to a proper AS, server, gateway, network function, network entity or alike.
It has also been found that when a Presence List Server (PLS) subscribes to the presence information of presentities, the routing done according to the current 3GPP IMS standard is not optimal. In addition, when the PLS (AS) initiates a request by itself, it is not defined how the PLS (AS) selects an S-CSCF.
There exists a problem that if a group server is seen as an application server, an ISC interface should be used. This has the disadvantage that routing is complicated in that an S-CSCF is needed in both the terminating and originating cases.
Another problem is that in known arrangements, the application server has to store all used service identities into an SLF, an HSS and/or another subscriber database.